The present invention relates to organic photodiodes and to their use in two dimensional image sensors. In more preferred embodiments it concerns organic photodiodes which are voltage switchable and which may be arrayed as image sensors in the form of a column-row (x-y) passively addressable matrix, where the x-y addressable organic image sensors (image arrays) have full-color or selected-color detection capability.
The development of image array photodetectors has a relatively long history in the device industry. Early approaches to imaging technology included devices based on thermal effects in solid state materials. These were followed by high sensitivity image arrays and matrices based on photodiodes and charge coupling devices (xe2x80x9cCCDsxe2x80x9d) made with inorganic semiconductors.
Photodiodes made with inorganic semiconductors, such as silicon, represent a class of high quantum yield, photosensitive devices. They have been used broadly in visible light detection applications in the past decades. However, they characteristically present a flat photocurrent voltage response, which makes them difficult to be used for fabricating high pixel density, x-y matrix addressable passive image sensors. An xe2x80x9cx-yxe2x80x9d matrix is a two dimensional array with a first set of electrodes perpendicular to a second set of electrodes. When passive devices such as resistors, diodes or liquid crystal cells are used as the pixel elements at the intersection points, the matrix is often called a xe2x80x9cpassivexe2x80x9d matrix (in contrast to an xe2x80x9cactivexe2x80x9d matrix in which active devices, such as thin film transistors, are used to control the turn-on for each pixel).
To effectively address each pixel from the column and row electrodes in a passive matrix, the pixel elements must exhibit strongly nonlinear current-voltage (xe2x80x9cI-Vxe2x80x9d) characteristics or an I-V dependence with a threshold voltage. This requirement provides the foundation for using light-emitting diodes or liquid crystal cells to construct passive x-y addressable displays. However, since the photoresponse of inorganic photodiodes is voltage-independent in reverse bias, photodiodes made with inorganic semiconductor crystals are not practical for use in high pixel-density, passive image sensorsxe2x80x94there is too much cross-talk between pixels. To avoid cross-talk, existing two dimensional photodiode arrays made with inorganic photodiodes must be fabricated with each pixel wired up individually, a laborious and costly procedure. In the case of such individual connections, the number of input/output leads is proportional to the number of the pixels. The number of pixels in commercial two dimensional photodiode arrays is therefore limited to xe2x89xa616xc3x9716=256 due to the difficulties in manufacturing and in making inter-board connections. Representative commercial photodiode arrays include the Siemens KOM2108 5xc3x975 photodiode array, and the Hamamatsu S3805 16xc3x9716 Si photodiode array.
The development of CCDs provided an additional approach toward high pixel density, two-dimensional image sensors. CCD arrays are integrated devices. They are different than x-y addressable passive matrix arrays. The operating principle of CCDs involves serial transfer of charges from pixel to pixel. These interpixel transfers occur repeatedly and result in the charge migrated, eventually, to the edge of the array for read-out. These devices employ super-large integrating circuit (xe2x80x9cSLICxe2x80x9d) technology and require an extremely high level of perfection during their fabrication. This makes CCD arrays costly (xcx9c$103-104 for a CCD of 0.75xe2x80x3-1xe2x80x3 size) and limits commercial CCD products to sub-inch dimensions.
The thin film transistor (xe2x80x9cTFTxe2x80x9d) technology on glass or quartz substrates, which was developed originally for the needs of liquid crystal displays, can provide active-matrix substrates for fabricating large size, x-y addressable image sensors. A large size, full color image sensor made with amorphous silicon (a-Si) p-i-n photocells on a-Si TFT panels was demonstrated recently [R. A. Street, J. Wu, R. Weisfield, S. E. Nelson and P. Nylen, Spring Meeting of Materials Research Society, San Francisco, Apr. 17-21 (1995); J. Yorkston et al., Mat. Res. Soc. Sym. Proc. 116, 258 (1992); R. A. Street, Bulletin of Materials Research Society 11(17), 20 (1992); L. E. Antonuk and R. A. Street, U.S. Pat. No. 5,262,649 (1993); R. A. Street, U.S. Pat. No 5,164,809 (1992)]. Independently, a parallel effort on small size, active-pixel photosensors based on CMOS technology on silicon wafers has been re-activated following developments in the CMOS technology with submicron resolution [For a review of recent progress, see: Eric J. Lerner, Laser Focus World 32(12) 54, 1996]. This CMOS technology allows the photocells to be integrated with the driver and the timing circuits so that a mono-chip image camera can be realized.
CCDs, a-Si TFTs, and active-pixel CMOS image sensors represent the existing/emerging technologies for solid state image sensors. However, because of the costly processes involved in fabrication of these sophisticated devices, their applications are severely limited. Furthermore, the use of SLIC technologies in the fabrication processes limit the commercial CCDs and the active-pixel CMOS sensors to sub-inch device dimensions.
Photodiodes made with organic semiconductors represent a novel class of photosensors with promising process advantages. Although there were early reports, in the 1980s. of fabricating photodiodes with organic molecules and conjugated polymers, relatively small photoresponse was observed [for an review of early work on organic photodiodes. see: G. A. Chamberlain, Solar Cells 8, 47 (1983)]. In the 1990s, there has been progress using conjugated polymers as the active materials; see for example the following reports on the photoresponse in poly(phenylene vinylene), PPV, and its derivatives,: S. Karg, W. Riess, V. Dyakonov, M. Schwoerer, Synth. Metals 54, 427 (1993); H. Antoniadis, B. R. Hsieh, M. A. Abkowitz, S. A. Jenekhe, M. Stolka, Synth. Metals 64, 265 (1994); G. Yu, C. Zhang, A. J. Heeger, Appl. Phys. Lett. 64,1540 (1994); R. N. Marks, J. J. M. Halls, D. D. D. C. Bradley, R. H. Frield, A. B. Holmes, J. Phys.: Condens. Matter 6, 1379 (1994); R. H. Friend, A. B. Homes, D. D. C. Bradley, R. N. Marks, U.S. Pat. No. 5,523,555 (1996)]. Recent progress has demonstrated that the photosensitivity in organic photodiodes can be enhanced under reverse bias. A photosensitivity of xcx9c90 mA/Watt was observed in ITO/MEH-PPV/Ca thin film devices at 10 V reverse bias (430 nm), corresponding to a quantum efficiency of  greater than 20% el/ph [G. Yu, C. Zhang and A. J. Heeger, Appl. Phys. Lett. 64, 1540 (1994); A. J. Heeger and G. Yu, U.S. Pat. No. 5,504,323 (Apr. 2, 1996)]. In photodiodes fabricated with poly(3-octylthiophene), photosensitivity over 0.3 A/Watt was observed over most of the visible spectral range at xe2x88x9215 V bias [G. Yu, H. Pakbaz and A. J. Heeger, Appl. Phys. Lett. 64, 3422 (1994)].
The photosensitivity in organic semiconductors can be enhanced by excited-state charge transfer; for example, by sensitizing the semiconducting polymer with acceptors such as C60 or its derivatives [N. S. Sariciftci and A. J. Heeger, U.S. Pat. No. 5,331,183 (Jul. 19, 1994); N. S. Sariciftci and A. J. Heeger, U.S. Pat. No. 5,454,880 (Oct. 3, 1995); N. S. Sariciftci, L. Smilowitz, A. J. Heeger and F. Wudl, Science 258, 1474 (1992); L. Smilowitz, N. S. Sariciftci, R. Wu, C. Gettinger, A. J. Heeger and F. Wudl, Phys. Rev. B 47, 13835 (1993); N. S. Sariciftci and A. J. Heeger, Intern. J. Mod. Phys. B 8, 237 (1994)]. Photoinduced charge transfer prevents early time recombination and stabilizes the charge separation, thereby enhancing the carrier quantum yield for subsequent collection [B. Kraabel, C. H. Lee. D. McBranch, D. Moses, N. S. Sariciftci and A. J. Heeger, Chem. Phys. Lett. 213, 389(1993); B. Kraabel. D. McBranch, N. S. Sariciftci, D. Moses and A. J. Heeger, Phys. Rev. B 50, 18543 (1994); C. H. Lee, G. Yu, D. Moses, K. Pakbaz, C. Zhang, N. S. Sariciftci, A. J. Heeger and F. Wudl, Phys. Rev. B. 48, 15425 (1993)]. By using charge transfer blends as the photosensitive materials in photodiodes, external photosensitivity of 0.2-0.3 A/W and external quantum yields of 50-80% el/ph have been achieved at 430 nm at low reverse bias voltages [G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science 270, 1789 (1995); G. Yu and A. J. Heeger, J. Appl. Phys. 78, 4510(1995); J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Frield, S. C. Moratti and A. B. Holmes, Nature 376, 498 (1995)]. At the same wavelength, the photosensitivity of the UV-enhanced silicon photodiodes is xcx9c0.2 A/Watt, independent of bias voltage [S. M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981) Part 5]. Thus, the photosensitivity of thin film photodiodes made with polymer charge transfer blends is comparable to that of photodiodes made with inorganic semiconducting crystals. In addition to their high photosensitivity, these organic photodiodes show large dynamic range; relatively flat photosensitivity has been reported from 100 mW/cm2 down to nW/cm2; i.e., over eight orders of magnitude [G. Yu, H. Pakbaz and A. J. Heeger, Appl. Phys. Lett. 64, 3422 (1994); G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science 270, 1789 (1995); G. Yu and A. J. Heeger, J. Appl. Phys. 78, 4510 (1995)]. The polymer photodetectors can be operated at room temperature, and the photosensitivity is relatively insensitive to the operating temperature, dropping by only a factor of 2 from room temperature to 80 K [G. Yu, K. Pakbaz and A. J. Heeger, Appl. Phys. Lett. 64, 3422 (1994)].
As is the case for polymer light-emitting devices [G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A. J. Heeger, Nature 357, 477 (1992); A. J. Heeger and J. Long, Optics and Photonics News, Aug. 1996, p.24], high sensitivity polymer photodetectors can be fabricated in large areas by processing from solution at room temperature, they can be made in unusual shapes (e.g. on a hemisphere to couple with an optical component or an optical system), or they can be made in flexible or foldable forms. The processing advantages also enable one to fabricate the photosensors directly onto optical fibers. Similarly, organic photodiodes can be hybridized with optical devices or electronic devices, such as an integrated circuits on a silicon wafer. These unique features make organic photodiodes special for many novel applications.
We have now found a new application for the enhancement of photosensitivity in organic photodiodes by applying a reverse bias.
We have now found that this variable photosensitivity enables an on-off voltage-switchable photosensors. At a reverse bias, typically greater than 1 V and more typically in the range of 2-15 V, the photodiode can be switched on with photosensitivity above 1 mA/W and particularly 10-500 mA/W or of 30-300 mA/W. The photosensitivity at a voltage close to the internal (built-in) potential is several orders of magnitude lower, equivalent to zero at the output of a 8-16 bit digital read-out circuit. This near zero state can thus be defined as the off state of the photodiode.
These voltage-switchable, organic photodiodes can serve as individual pixels in passive diode arrays. These arrays can be in the form of x-y addressable arrays with anodes connected via row (column) electrodes and cathodes connected via column (row) electrodes. Every pixel can be selected, and the information (intensity of the incident light) at each pixel can be read out without crosstalk.
These arrays can utilize the processing advantages associated with the fabrication of organic diode structures from soluble, semiconducting, conjugated polymers (and/or their precursor polymers). Layers of these materials can be cast from solution to enable the fabrication of large active areas, onto substrates with desired shapes. This also enables active areas to be in flexible form. These photoactive materials can be patterned onto an optically uniform substrate by means of photolithography, microcontact printing, shadow masking and the like. For the photosensors for visible light sensing applications, the substrate can be opaque for xcex less than 400 nm so that the pixels are insensitive to UV radiation.
The photoactive layer employed in these switchable photodiodes is made up of organic materials. These take numerous forms. They can be conjugated semiconducting polymers and blends of polymers including such materials. Donor-acceptor blends, with conjugated polymers serving as the donor and polymeric and monomeric acceptors can also be used as can molecular donors and acceptors (i.e. molecules rather than macromolecules) well known in the art. Examples of the latter include anthracene and its derivatives, pinacyanol and its derivatives thiophene oligomers (such as sexithiophene.6T, and octylthiophene, 8T) and their derivatives and the like, phenyl oligomers (such as sexiphenyl or octylphenyl) and the like. In addition, one can employ multiple layers of organic semiconducting materials in donor/acceptor heterojunction configurations. In addition, one can employ multiple layers of organic semiconducting materials in donor/acceptor, heterojunction configurations.
The organic image sensors enabled by this invention can have mono-color or multi-color detection capability. In these image sensors color selection can be achieved by combining a suitable color filter panel with the organic image sensors and image sensor arrays already described. If desired, the color filter panel can serve as a substrate upon which the image sensor is carried.
In addition, embodiments of the present invention provide organic image sensors with full-color detection capability. In these organic image sensors, a filter panel is made up of red, green and blue color filters which are patterned in a format corresponding to the format of a photodiode array. The panel of patterned filters and the patterned photodiode array are coupled (and coordinated) such that a colored image sensor is formed. The patterned color filter panel can be used directly as the substrate of the image sensor.
Full-color detectivity, is also achieved when red, green and blue are detected by three of these photodiodes with spectral response cut-off at 500 nm, 600 nm and 700 nm, respectively. Differentiation operations in the read-out circuit extract the red (600-700 nm), green (500-600 nm) and blue (400-500 nm) signals.
In addition to be used in their intrinsic response spectral ranges (typically between 200 nm-1000 nm), these organic photodiodes can be used for photosensing in other spectral ranges such as in ultraviolet (UV), in X-ray and in infrared (IR) spectra. The photosensing of photon energies at other than their intrinsic spectral ranges can be achieved via techniques of energy down or up conversion For example, photosensing in UV and X-ray spectral ranges can be achieved with a phosphor layer or a scintillation layer placed betwen the UV or X ray source and the photosensor. The layer will convert the high energy radiation into visible emission which is detected be the organic photosensors. IR radiation and high energy particle radiation can be detected following similar principles.
These organic photosensors are also sensitive intrinsically to radiation at specific wavelengths in deep UV and X-ray spectral ranges. These wavelengths and their corresponding photon energies are related to optical transitions from inner core levels (energy bands) to the lowest unoccupied molecular orbital (LUMO) or the bottom of the conduction band.